feat: exports polling utilities from `@dfinity/agent`

[krpeacock]

Description

Noted by @peterpeterparker - the agent doesn't export the utilities out of the polling submodule.

At the same time, I've set the default polling strategy as an actual default, and the argument can now be omitted

Checklist:

• [X] My changes follow the guidelines in CONTRIBUTING.md.
• [X] The title of this PR complies with Conventional Commits.
• [X] I have edited the CHANGELOG accordingly.
• [X] I have made corresponding changes to the documentation.

[github-actions[bot]]

size-limit report 📦

Path	Size
@dfinity/agent	85.96 KB (+0.01% 🔺)
@dfinity/candid	13.58 KB (0%)
@dfinity/principal	4.97 KB (0%)
@dfinity/auth-client	60.36 KB (0%)
@dfinity/assets	80.98 KB (+0.01% 🔺)
@dfinity/identity	57.56 KB (0%)
@dfinity/identity-secp256k1	131.14 KB (+0.11% 🔺)