rpc (fix): Use lazy val for generated RPCMethod to avoid xxxInternals$.<clinit> is too large error

[xerial]

This error happened in Scala 3 when compiling a RPC client for a large API

[codecov]

Codecov Report

Merging #3155 (9afaf29) into main (45198a1) will not change coverage.
Report is 3 commits behind head on main.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3155   +/-   ##
=======================================
  Coverage   82.88%   82.88%           
=======================================
  Files         350      350           
  Lines       14743    14743           
  Branches     2447     2447           
=======================================
  Hits        12219    12219           
  Misses       2524     2524           

Files Changed	Coverage Δ
...frame/http/codegen/client/RPCClientGenerator.scala	100.00% <ø> (ø)

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b6ab427...9afaf29. Read the comment docs.