Balancer pause helper

[elshan-eth]

Description

A helper to stop pools. Only pools that are added to the list can be stopped. The Hypernative keeper must be added to Authentication to access the PauseHelper. The PauseHelper must be added to Authentication to access the Vault pausePool.

Type of change

• Bug fix
• New feature
• Breaking change
• Dependency changes
• Code refactor / cleanup
• Optimization: [ ] gas / [ ] bytecode
• Documentation or wording changes
• Other

Checklist:

• The diff is legible and has no extraneous changes
• Complex code has been commented, including external interfaces
• Tests have 100% code coverage
• The base branch is either main, or there's a description of how to merge

Issue Resolution

Closes #1209

[EndymionJkb]

First pass; didn't look at the tests yet.

[EndymionJkb]

Shaping up; some naming and style suggestions. Some comments from earlier might have gotten lost.

[joaobrunoah]

LGTM!

[EndymionJkb]

Agree it would be good to also define an interface; we usually do for these kinds of contracts. Also had a note on potentially renaming to PausePoolHelper, and some function/comment suggestions

[EndymionJkb]

Do we also want a getPoolAt(uint256 index)?
Could also have a getPools() returns (address[] memory pools) that just returns them all.

That way it supports 3 ways of using it:

1. simple getPools() if you know there isn't a pagination issue;
2. generic iteration: for(i = 0; i < getPoolsCount(); ++i) { address pool = getPoolAt(i); }
3. pagination if needed, using getPools(from, to);

[elshan-eth]

I couldn't come up with a reason why getPoolAt(uint256 index) might be needed. It seems unnecessary to me.

As for getPools(), I think that if it can potentially break at some point, it's better not to implement such a function.