Fix incorrect logtype encoding by properly escaping variable placeholder characters (fixes #163).

[LinZhihao-723]

Reference

#163

Description

When creating the logtype dictionary, the characters of the variable placeholders in the original log events are not properly escaped, which causes #163. This PR fixes this problem by properly escaping these characters and tracking the location of the escaped placeholders for each logtype dictionary entry. It also handles the escape in ffi related code.
Rules we added in this PR for logtype encoding:

1. Variable placeholder characters are escaped when encoding a logtype for compression.
2. Variable placeholder characters are double escaped when building a search query. This is required for the wildcard
3. When building a query, if the input contains an escaped wildcard character, we should not escape the escape character.

Validation performed

• Validated File with messages which swap a variable for a variable placeholder aren't compressed correctly #163 was fixed by following its reproduction steps
• Ran unit tests and modified the unit tests to cover the problem this PR resolved

[kirkrodrigues]

We also need to update the logtype query generation in ffi::search. That instance is a bit more complicated: If the logtype query contains no wildcards, we don't want to escape the escape characters (double-escape) since the logtype query won't be used in wildcard matches. I guess the most straightforward way to do this would be to do it while searching the logtype for wildcards in the Subquery constructor.