fixing up ech examples

[sebastian-carpenter]

Main problem was tls/client-ech.c which had an expired certificate and lacked error checking for many function calls. It also GREASED against the private SNI rather than the public SNI which defeats the purpose of using ECH in the first place (since the GREASE would expose the private SNI).

client-ech:

• updated the cert
• updated example to more closely follow the layout given by other examples
• The ip address seems to have changed as well so I updated that too
• Changed GREASE to run against public SNI rather than private SNI.

client-ech-grease:

• Added standalone grease example to be used against the local server server-ech-local

others:

• changed errors messages, whitespace, ...

Updated README.md to contain info about ECH examples and some info about ECH.