bitreq: test all TLS feature combinations in CI

[satsfy]

Closes #653

The test job only built bitreq with default features, so the optional TLS backends were never compiled. List each backend under rbmt exact_features so everyone gets built and tested.

The sync backends also need std in their set, or nothing is tested (cargo test -p bitreq --no-default-features --features https-rustls --test main -> running 0 tests).

rustls alone is excluded because it has no trust roots (say, use it with webpki-roots), so test_https fails (I think this test gate should change, but that is follow-up territory).

Using this required bumping rbmt to 0.5.1 (it was on 0.2.0).

Note that this makes the CI run for longer.

[satsfy]

After inspecting the logs I have noticed that the amount of ran features has not changed at all. I think CI already handled all jobs by default. Will draft until resolved.

[nyonson]

At the very end of a test job, for example stable/minimal, rbmt prints a summary. But I think the issue here is that the sample_strategy was added in 0.4.0.

Example from this PR

 [1480](https://github.com/rust-bitcoin/corepc/actions/runs/29347149288/job/87133726541?pr=662#step:5:11481)

Test Summary
Commit: 33ed9f25b81edcca565f758fdcc93665c7f96ef1
Package: corepc-client
  Examples           : (none)
  Individual features: client-sync, jsonrpc
  Sampled subsets    : [client-sync]
  Exact sets         : (none)
  No-std check       : skipped

Package: jsonrpc
  Examples           : (none)
  Individual features: base64, bitreq, bitreq_http, bitreq_http_async, client_async, proxy, simple_http, simple_tcp, simple_uds, socks
  Sampled subsets    : [base64, bitreq, bitreq_http, client_async, proxy, simple_http, simple_tcp, simple_uds, socks], [bitreq, client_async, simple_http, simple_tcp, socks], [base64, bitreq_http_async, proxy, simple_http, simple_uds, socks], [base64, proxy, simple_http, simple_tcp, simple_uds]
  Exact sets         : (none)
  No-std check       : skipped

Package: bitreq
  Examples           : (none)
  Individual features: async, async-https, async-https-native-tls, async-https-rustls, async-https-rustls-probe, base64, https, https-native-tls, https-rustls, https-rustls-probe, json-using-serde, log, native-tls, proxy, rustls, rustls-native-certs, rustls-webpki, serde, serde_json, std, tokio, tokio-native-tls, tokio-rustls, webpki-roots
  Sampled subsets    : [async-https-native-tls, async-https-rustls, base64, log, proxy, rustls-webpki, webpki-roots], [async, https, https-rustls, log, native-tls, rustls-native-certs, rustls-webpki, tokio-native-tls], [async, async-https, async-https-native-tls, async-https-rustls, base64, https-native-tls, https-rustls, https-rustls-probe, json-using-serde, log, native-tls, proxy, rustls, serde, serde_json, tokio, tokio-native-tls, tokio-rustls], [async-https, base64, https-native-tls, https-rustls, https-rustls-probe, json-using-serde, log, native-tls, proxy, rustls-native-certs, rustls-webpki, serde, serde_json, std, tokio-native-tls, webpki-roots], [async, async-https-rustls, https-native-tls, https-rustls, log, native-tls, proxy, rustls, rustls-native-certs, serde, tokio]
  Exact sets         : (none)
  No-std check       : skipped

Package: corepc-types
  Examples           : (none)
  Individual features: serde-deny-unknown-fields, std
  Sampled subsets    : (none)
  Exact sets         : (none)
  No-std check       : skipped

Package: jsonrpc-fuzz
  Examples           : (none)
  Individual features: (none)
  Sampled subsets    : (none)
  Exact sets         : (none)
  No-std check       : skipped

[satsfy]

Reworked the PR: bumped rbmt to 0.5.1 (because sample_strategy needs 0.4.0+), dropped sample_strategy = all for an explicit exact_features list so each TLS backend actually gets tested.