Add default std feature, prep for 0.28.0 release

[notmandatory]

Description

Add new std feature which enables bitcoin/std and miniscript/std, and enable std feature in default feature set.

Notes to the reviewers

When using bdk in a wasm environment then default features should be disabled and bitcoin/no-std and miniscript/no-std must be enabled. This is a breaking change for anyone using bdk with --no-default-features.

I also updated the workflows to also run for release/* branches in addition to master.

Changelog notice

Changed

• Added new std feature as part of default features, std must be enabled unless building for wasm.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

[rajarshimaitra]

ACK cbcbdd1

I tested the builds with a few regular combinations. Looks good to me.