feat: initial version of module spring-testcontainer-cln-starter

[theborakompanioni]

Adds a core lightning grpc client and a testcontainer to use in regtest mode.

• add properties for certs and keys
• remove unused/dead code, fix javadocs (e.g. host in ClnClientAutoConfigProperties)
• add showcase of grpc client to example application

Not done in this issue:

• Actuator endpoints cln: add actuator endpoints #61
• create SslContext cln: add ssl context to auto config #62
  • probably need to have a hard dependency on io.grpc:grpc-netty-shaded (see SSL Related Methods in ManagedChannelBuilder  grpc/grpc-java#3556 - still not resolved? Needs investigation)
  • Bonus: Add AutoConfiguration for client when ClnTestcontainer bean is present (only when in regetest and optionally disable with flag)