chore: add example how to install additional rpc namespace

[mattsse]

Closes #4001

Add an example that shows how to extend the reth default CLI with custom rpc namespace

this also adds a few reexport mods for convenience

[codecov]

Codecov Report

Merging #4005 (dde3fc5) into main (dee14c7) will decrease coverage by 0.05%.
Report is 1 commits behind head on main.
The diff coverage is 0.00%.

Files Changed	Coverage Δ
bin/reth/src/cli/mod.rs	34.40% <0.00%> (ø)
bin/reth/src/node/mod.rs	12.06% <ø> (-0.12%)	⬇️
crates/rpc/rpc-builder/src/lib.rs	63.59% <0.00%> (-0.57%)	⬇️

... and 15 files with indirect coverage changes

Flag	Coverage Δ
integration-tests	16.23% <0.00%> (-0.03%)	⬇️
unit-tests	64.14% <0.00%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
reth binary	25.63% <0.00%> (-0.07%)	⬇️
blockchain tree	83.04% <ø> (ø)
pipeline	90.03% <ø> (ø)
storage (db)	74.30% <ø> (ø)
trie	94.70% <ø> (ø)
txpool	45.40% <ø> (-0.61%)	⬇️
networking	77.51% <ø> (ø)
rpc	58.32% <0.00%> (-0.04%)	⬇️
consensus	63.51% <ø> (ø)
revm	33.10% <ø> (ø)
payload builder	6.58% <ø> (ø)
primitives	88.06% <ø> (+0.01%)	⬆️

[Rjected]

lgtm, the example is simple + docs are informative