Add frozen_string_literal: true in order to suppress 'warning: literal string will be frozen in the future'

[Watson1978]

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:
To supress warning: literal string will be frozen in the future,
we will require to fix with following proposals.

1. Add # frozen_string_literal: true in the beginning of the file
2. Add # frozen_string_literal: false in the beginning of the file
3. Add RUBYOPT="-disable=frozen_string_literal" as launch option

These suggestions may be combined.

This PR adds # frozen_string_literal: true to all required files.
Seems CI will be passed,
however, Fluentd might raise FrozenError exception by unexpected frozen string
if it has not sufficient test suites...

Related to GH-4585

Docs Changes:

Release Note:

[ashie]

Thanks for clarifying the issue!
Once Ruby's direction is clearer, we'll determine on our direction.