[exporter/datadog] trim whitespace from hostname

[chandan009s]

Description

When the Datadog exporter hostname is configured using the ${file:} provider,
the trailing newline from the file is preserved and causes hostname validation
to fail.

This change trims surrounding whitespace from the configured hostname during
unmarshalling so file-based hostnames such as my-hostname\n are accepted.

Link to tracking issue

Fixes #50935

Testing

Added a regression test covering a hostname with a trailing newline.

Tests run:

• go test ./... in pkg/datadog/config

Documentation

No documentation changes.

Authorship

• I, a human, wrote this pull request description myself.

---

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: chandan009s / name: chandan009s (773c73b)

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