MINOR: server: Add parser support for set-proxy-v2-tlv-fmt

MINOR: server: Add parser support for set-proxy-v2-tlv-fmt

This commit introduces a generic server-side parsing of type-value pair
arguments and allocation of a TLV list via a new keyword called
set-proxy-v2-tlv-fmt.

This allows to 1) forward any TLV type with the help of fc_pp_tlv,
2) generally, send out any TLV type and value via a log format expression.
To have this fully working the connection will need to be updated in
a follow-up commit to actually respect the new server TLV list.

default-server support has also been implemented.