add ability to remove dns-prefetch or preconnect link tags

[tdatu]

Issue: #1022

Approach

Implemented apply_filters in setup_hooks

QA notes

Simple hooks and implemented in test environment.

[atdcloud]

@tharsheblows, please check.

[tharsheblows]

@atdcloud I've left a comment -- it's not necessarily a blocker but will make the filter a bit easier to use and understand.

Is there somewhere where we should add documentation about the filter?

[tharsheblows]

📝 note
This will work but I think the way to do it is something like below. It keeps it a bit more understandable when looking at the code.

... 
$dns_prefetch_types = apply_filters( 'cld_dns_prefetch_types', array ( 'dns-prefetch', 'preconnect' ) );

if ( in_array( $relation_type, $dns_prefetch_types, true ) { 
...

[tdatu]

hmmm...I think the IF clause that evaluates the $relation_type is already a dead giveaway.

[dero]

I think we should do a couple of things here:

1. Avoid using the words prefetch_types in filter and variable names, as it's a misnomer. Let's use resource_hints.
2. I support using in_array to explicitly search through an array instead of mutating $relation_type. In fact, mutating it has no purpose, as the $relation_type sets the context for the entire method and should, in my opinion, be treated as immutable.

...
$resource_hints = apply_filters( 'cld_resource_hints', array ( 'dns-prefetch', 'preconnect' ) );

if ( in_array( $relation_type, $resource_hints, true ) { 
...

[dero]

... and we need to add a hook comment above the new hook in order to make sure the generated documentation is correctly created.