Fix type hint on prefix argument to AgentCheck.normalize()

[florimondmanca]

What does this PR do?

Fix type hint on the prefix argument to AgentCheck.normalize(), as it actually handles both text and binary.

Also adds type annotations to ensure_bytes() and ensure_unicode() to fully type the .normalize() implementation.

Cleanup for #5965

Motivation

Mypy was improperly flagging errors in the SNMP check:

integrations-core/snmp/datadog_checks/snmp/snmp.py

Line 505 in 27ca9e5

metric_name = self.normalize(name, prefix='snmp')

error: Argument "prefix" to "normalize" of "AgentCheck" has incompatible type "str"; expected "Optional[bytes]

Additional Notes

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached

[codecov]

Codecov Report

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The diff coverage is 100%.

Impacted Files	Coverage Δ
...dog_checks_base/datadog_checks/base/checks/base.py	84.57% <100%> (ø)
...og_checks_base/datadog_checks/base/utils/common.py	90.38% <100%> (ø)