rpc: expose http::Builder, websocket::Builder under client

[mzabaluev]

Re-export client::transport::{http, websocket} (guarded by appropriate feature gates) publicly under the client module.
This exposes the Builder types defined in each of the modules as public API, with properly built documentation.

• Referenced an issue explaining the need for the change
• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Added entry in .changelog/

[codecov-commenter]

Codecov Report

Merging #1378 (fdf8f64) into main (37822e5) will decrease coverage by 0.7%.
Report is 2 commits behind head on main.
The diff coverage is n/a.

❗ Current head fdf8f64 differs from pull request most recent head 79ee9d0. Consider uploading reports for the commit 79ee9d0 to get more accurate results

@@           Coverage Diff           @@
##            main   #1378     +/-   ##
=======================================
- Coverage   59.7%   59.0%   -0.7%     
=======================================
  Files        275     275             
  Lines      27597   27930    +333     
=======================================
+ Hits       16485   16491      +6     
- Misses     11112   11439    +327     

Files	Coverage Δ
light-client-verifier/src/errors.rs	3.3% <ø> (ø)
rpc/src/client.rs	4.5% <ø> (ø)

... and 4 files with indirect coverage changes

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