feat: BTC explorer network config

[He1DAr]

Introduce new environment variables to configure BTC explorer base URL:

NEXT_PUBLIC_MAINNET_BTC_BLOCK_BASE_URL (defaults to 'https://mempool.space/block')
NEXT_PUBLIC_MAINNET_BTC_TX_BASE_URL (defaults to 'https://mempool.space/tx')
NEXT_PUBLIC_MAINNET_BTC_ADDRESS_BASE_URL (defaults to 'https://mempool.space/address')

NEXT_PUBLIC_TESTNET_BTC_BLOCK_BASE_URL (defaults to 'https://mempool.space/testnet/block')
NEXT_PUBLIC_TESTNET_BTC_TX_BASE_URL (defaults to 'https://mempool.space/testnet/tx')
NEXT_PUBLIC_TESTNET_BTC_ADDRESS_BASE_URL (defaults to 'https://mempool.space/testnet/address')

Add BTC explorer fields to custom network form:

closes #1296

[vercel]

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Name	Status	Preview	Comments	Updated (UTC)
hiro-explorer	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Oct 13, 2023 1:05pm

[codecov]

Codecov Report

Merging #1306 (6912233) into main (47c86dd) will increase coverage by 0.16%.
Report is 3 commits behind head on main.
The diff coverage is 39.28%.

@@            Coverage Diff             @@
##             main    #1306      +/-   ##
==========================================
+ Coverage   23.95%   24.11%   +0.16%     
==========================================
  Files         454      456       +2     
  Lines        6671     6730      +59     
  Branches     1706     1735      +29     
==========================================
+ Hits         1598     1623      +25     
- Misses       4907     4938      +31     
- Partials      166      169       +3     

Files	Coverage Δ
src/app/Providers.tsx	0.00% <ø> (ø)
src/common/constants.ts	88.46% <100.00%> (+3.46%)	⬆️
src/common/constants/network.ts	100.00% <100.00%> (ø)
src/common/types/network.ts	100.00% <ø> (ø)
src/components/btc-stx-block-links.tsx	78.26% <100.00%> (-1.74%)	⬇️
src/components/modals/AddNetwork/utils.ts	100.00% <100.00%> (ø)
src/app/txid/[txid]/Cards/BtcAnchorBlockCard.tsx	0.00% <0.00%> (ø)
src/components/app-config.tsx	0.00% <0.00%> (ø)
src/components/function-summary/value.tsx	44.00% <0.00%> (-0.24%)	⬇️
src/components/modals.tsx	0.00% <0.00%> (ø)
... and 8 more

... and 1 file with indirect coverage changes

[blockstack-devops]

🎉 This PR is included in version 1.126.0 🎉

The release is available on GitHub release

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