header_rewrite: Add set-plugin-cntl

[maskit]

I was surprised that %{NOW} in header rewrite rules does not always use GMT.

trafficserver/doc/admin-guide/plugins/header_rewrite.en.rst

Lines 560 to 565 in ce7287b

	cond %{NOW:<part>} <operand>
	This is the current time, in the local timezone as set on the machine,
	typically GMT. Without any further qualifiers, this is the time in seconds
	since EPOCH aka Unix time. Qualifiers can be used to give various other
	values, such as year, month etc.

The new operator, set-plugin-cntl, enables users to change fundamental behavior of header_rewrite plugin. I added TIMEZONE controller for now. You can use it like below to use GMT regardless of system setting.

cond %{SEND_RESPONSE_HDR_HOOK}
  set-plugin-cntl TIMEZONE GMT
  set-header hour %{NOW:HOUR}

I made this new operator applicable for other things because there's at least one more thing I want to change.

[maskit]

tls_conn_timeout failed

[maskit]

[approve ci autest 0]

[bryancall]

NOW is not a valid timezone, maybe put LOCAL there instead?

[maskit]

NOW is not a default value. There are three columns and the second column shows the operators and conditions that are controlled by a controller.

[bryancall]

LOCAL and GMT look like the only two possible values, should add that to the docs.

[maskit]

It's already written on the docs.