[track] Release subxt to crates.io

[dvdplm]

Tracking ticket to release subxt as a crate on crates.io.

Things we want in:

• Cannot use my own Config type to instantiate an API #391 (Possible bug preventing anything other than DefaultConfig being used) Parameterize AccountData #409
• Handle BitVec decoding in Events #405
• Prioritize a transaction by tip #187
• Remove pub from various fields in exported types, notably in extrinsic::extra #390 (I think that it would be good to do this tidy up prior to a release)
• Refactor duplication for generation of composite fields #328 Refactor type generation, remove code duplication #352
• Change storage key arguments (and call arguments?) to being references #411
• Feature request: better error matching support #313 (investigate)
• Follow ups to https://github.com/paritytech/subxt/pull/294 #309 (TODO: split up into single issues)
• Use released substrate dependencies #375 (and resolve issue with mismatching DispatchError Use the generated DispatchError instead of the hardcoded Substrate one #394)
• Configurable extrinsic Extra  #331 Configurable Config and Extra types #373
• Pin PR CI tests against specific substrate; daily compat tests against latest master. #334 (Not "done" but we have nightly runs now to test compatibility against latest)
• System properties should be optional #316 Make system properties an arbitrary JSON object, plus CI fixes #349
• Finer grained event subscription API  #312

Known blockers

• git dependencies

Closes #339

Nice-to-haves, work for future releases:

• Codegen output not implements codec::Decode #315
• Dynamically derive default mortality from System::Constants::BlockHashCount #206
• Optional start height for subscriptions #272
• Create FAQ to address common issues #393

[jsdw]

Note: add a deprecation notice to substrate-subxt 0.15 on crates.io when we release subxt 0.16