Add boolean baked-in validator to check a string value is a valid boolean representation

[xvello]

Add boolean baked-in validator

Due to technical limitations upstream, some of our structs can only have string fields, but some of these fields will be parsed into booleans by the business logic, using strconv.ParseBool:

ParseBool returns the boolean value represented by the string. It accepts 1, t, T, TRUE, true, True, 0, f, F, FALSE, false, False. Any other value returns an error.

Similar to the numeric validator, this PR adds a boolean validator that checks that the field will be safely convertible to a boolean. Instead of enumerating all values currently accepted by ParseBool, this PR just calls the method and confirms it does not return an error. The rationale for this is to stay in sync if the accepted values change in future versions of go.

I updated the tests and documentation, but I might have forgotten something else to update.

Make sure that you've checked the boxes below before you submit PR:

• Tests exist or have been written that cover this particular change.

@go-playground/validator-maintainers

[coveralls]

Coverage increased (+0.008%) to 75.1% when pulling 5ce08ba on xvello:feature/boolean into 14221d0 on go-playground:master.