feat: add UnsubscribeBackoff param

[twoeths]

Motivation

• We want to specify lower backoff time if unsubscribing from a topic as we may resubscribe from that same topic soon
• To follow v1.1 spec of gossipsub https://github.com/libp2p/specs/blob/master/pubsub/gossipsub/gossipsub-v1.1.md

Description

• In makePrune(), add a new parameter onUnsubscribe. If it's true specify the backoff time as UnsubscribeBackoff param, update all consumers accordingly
• Also found that we missed adding backoff in this function, double checked with rust-libp2p https://github.com/libp2p/rust-libp2p/blob/f335697573e2688390deeb9708abaeec9f0b9d9f/protocols/gossipsub/src/behaviour.rs#L1116

Closes #249

[codecov-commenter]

Codecov Report

Patch coverage: 73.52% and project coverage change: -0.14 :warning:

Comparison is base (cbdae04) 82.51% compared to head (ef9b2c1) 82.37%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #447 +/- ## ========================================== - Coverage 82.51% 82.37% -0.14% ========================================== Files 48 48 Lines 11966 12008 +42 Branches 1279 1280 +1 ========================================== + Hits 9874 9892 +18 - Misses 2092 2116 +24 ``` | [Impacted Files](https://app.codecov.io/gh/ChainSafe/js-libp2p-gossipsub/pull/447?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ChainSafe) | Coverage Δ | | |---|---|---| | [src/index.ts](https://app.codecov.io/gh/ChainSafe/js-libp2p-gossipsub/pull/447?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ChainSafe#diff-c3JjL2luZGV4LnRz) | `69.90% <68.96%> (-0.21%)` | :arrow_down: | | [src/constants.ts](https://app.codecov.io/gh/ChainSafe/js-libp2p-gossipsub/pull/447?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ChainSafe#diff-c3JjL2NvbnN0YW50cy50cw==) | `100.00% <100.00%> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/ChainSafe/js-libp2p-gossipsub/pull/447/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ChainSafe)

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