Add standalone a2acli CLI

[muscariello]

Closes #37

Summary

Adds a standalone A2A CLI as a new workspace crate, published as agntcy-a2acli and installed as the a2acli binary.

This PR includes:

• fetching the public agent card and extended agent card
• sending one-shot and streaming messages
• getting, listing, canceling, and subscribing to tasks
• creating, getting, listing, and deleting push notification configs
• bearer-token and custom-header request propagation
• workspace and crate documentation for install and usage
• a changelog entry for the new package

Companion fixes included in this branch:

• a2a-client now applies interceptor before/after hooks consistently around transport calls
• agent-card and agent-skill securityRequirements deserialization now accepts wrapped spec-style shapes for compatibility

Validation

• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test -p agntcy-a2acli
• LLVM_COV=$(xcrun --find llvm-cov) LLVM_PROFDATA=$(xcrun --find llvm-profdata) cargo llvm-cov -p agntcy-a2acli --summary-only

agntcy-a2acli coverage:

• 95.63% region coverage
• 98.95% line coverage

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.05037% with 24 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
a2a/src/agent_card.rs	83.52%	14 Missing ⚠️
a2acli/src/lib.rs	99.08%	9 Missing ⚠️
a2a-client/src/client.rs	99.30%	1 Missing ⚠️

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