Update the message matcher to improve the literal pattern match speed

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Current

Fields[environment.addons] =~ "pioneer%-opt%-in@mozilla%.org"

Proposed (add a modifier like in regex /foo/i)

Fields[environment.addons] =~ "[email protected]"%

Alternatives Considered

• add a new operator =%(the modifier is a better fit for future extensions)
• extend the existing =~ operator to detect if there are only escaping patterns in a match but this is more tedious for the user as the string still has to be fully escaped

Benefits

The speed of execution depends on the string lengths but for the telemetry case above a speed up of 2-3x is expected in the matcher. From the plugin perspective there will be a reduction in the maximum memory (estimate a 10x reduction) and a corresponding reduction in GC.

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2e13919

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The grammar was modified to only allow this operation on Headers