prompty: improve token count estimation

[ianchi]

Description

Current token count estimation is incomplete.
This PR adds tools definitions and tool calls into the computation, and allows to define a valid OpenAI model name to use to determine the tokenizer, which is needed in case of Azure OpenAI when the deployment is not a valid OpenAI model name.

Fixes #3809.

All Promptflow Contribution checklist:

• The pull request does not introduce [breaking changes].
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.
• I confirm that all new dependencies are compatible with the MIT license.
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General Guidelines and Best Practices

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Testing Guidelines

• Pull request includes test coverage for the included changes.

[github-actions]

Hi, thank you for your interest in helping to improve the prompt flow experience and for your contribution. We've noticed that there hasn't been recent engagement on this pull request. If this is still an active work stream, please let us know by pushing some changes or leaving a comment.

[ianchi]

Hi, any chance to advance with this?

[github-actions]

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