soroban-rpc: Add admin endpoint including pprof debug endpoints

[2opremio]

What

Add admin http endpoint containing pprof debug endpoints

It adds an --admin-endpoint flag (matching the normal --endpoint flag), which indicates where to listen.

For instance, when providing --admin-endpoint http://localhost:9000 the endpoints like the following will be exposed:

http://localhost:9000/debug/pprof/
http://localhost:9000/debug/pprof/cmdline
http://localhost:9000/debug/pprof/profile
http://localhost:9000/debug/pprof/symbol
http://localhost:9000/debug/pprof/trace
...

Depends on #566

Why

Closes #498 and paces the way for #496 which also needs an admin base endpoint

Known limitations

[TODO or N/A]

[2opremio]

@tsachiherman PTAL, can't merge without your confirmation.

[tsachiherman]

@tsachiherman PTAL, can't merge without your confirmation.

I blocked this one only for the nil issue I pointed out ( and you fixed ).