x-pack/filebeat/input/{azureblobstorage, gcs}: Remove leading BOM from JSON text

[kcreddy]

Proposed commit message

x-pack/filebeat/input/{azureblobstorage, gcs}: Remove leading BOM from JSON text

When parsing JSON leading BOMs are identified in azureblobstorage 
input. Currently the input cannot handle this and fails indicating a 
malformed JSON but without the position and characters leading 
to the failure. 

This PR handle the leading BOMs in JSON and removes them in 
azureblobstorage input. Also improve error reporting from malformed
JSON. Since a similar handling is observed in GCS input, the change 
is replicated there.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works. Where relevant, I have used the stresstest.sh script to run them under stress conditions and race detector to verify their stability.
• I have added an entry in ./changelog/fragments using the changelog tool.

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[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @kcreddy? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[elasticmachine]

Pinging @elastic/security-service-integrations (Team:Security-Service Integrations)