feat: Support runtime control of connection rate limiting via socket

[b1tamara]

The PR introduces process-level variables (for the properties block and connections) for the connection rate limit feature, enabling runtime control via the HAProxy Runtime API. It allows dynamic start/stop of rate limiting and on-the-fly threshold changes
Co-Authored-By: @Dariquest and @Mrizwanshaik

[hoffmaen]

Maybe make the two new proc. variables more prominent, by introducing them in an unsorted list, similar to the variables and options listed in the section "Configuration Options". It was first a bit hard for me to understand why we have these proc variables now.

One point about the naming:
connections_rate_limit.block -> proc.conn_rate_block
connections_rate_limit.connections -> proc.conn_rate_limit

Is inconsistent. If we could still change the it and accept it's breaking, I'd prefer the following variable naming:

connection_rate_limit.enabled -> proc.connection_rate_limit_enabled
connection_rate_limit.connections -> proc.connection_rate_limit_connections

This would be more consistent, and having enabled instead of block indicates that this is a boolean switch.

[b1tamara]

Renamed variables:
connection_rate_limit.block -> proc.connection_rate_limit_block
connection_rate_limit.connections -> proc.connection_rate_limit_connections

Enhanced the documentation

[mike-jc]

Did I understand correctly that the existence of connections_rate_limit.connections in manifest controls if haproxy config will have a process variable, and when it is not set in manifest, the rate-limit control is not possible on the fly? If so, it does not look straightforward to me.
Can we set the variable to zero if the parameter is not set in the manifest? Similar to what we do for proc.conn_rate_block.

Suggested change

	<%- if_p("ha_proxy.connections_rate_limit.connections") do |conn_rate_connections| -%>
	set-var proc.conn_rate_limit int(<%= conn_rate_connections %>)
	<%- end -%>
	set-var proc.conn_rate_limit int(<%= p("ha_proxy.connections_rate_limit.connections", 0) %>)

[b1tamara]

We store the value from the manifest in a process-level variable if defined. If not, it can be set via the socket later. We cannot set the default value to 0, as 0 is a very wrong threshold.