logging: Implement rename filter, changes field key names

[francislavoie]

Fixes #4744

Quick little feature; lets you rename the key of a field:

	log {
		format filter {
			wrap console
			fields {
				request>remote_ip rename client_ip
			}
		}
	}

Little implementation note. If you try to rename the same field twice, you must use the original name again; they can't be chained. E.g. this will not work, and the result will be client_ip:

request>remote_ip rename client_ip
request>client_ip rename something_else

But this will result in something_else:

request>remote_ip rename client_ip
request>remote_ip rename something_else

[mholt]

🚢 it

Neat contribution! Thank you.