Create a Reusable GetBlockExplorerUrl Util

[Douglashdaniel]

Description

Currently we have multiple methods through out the wallet for getting an EthereumChain block explorer URL.
We should create a reusable GetBlockExplorerUrl util to cut down on duplicated code.

Test plan

No functional change, so the test plan is to ensure block explorer links work correctly (see Demo in linked PR for an example).

• Should be able to view addresses, transactions, and contracts (whenever relevant).
• Should be able to open block explorer links both from Wallet page as well as panel.
• Should be able to open block explorer links for custom networks, such as Polygon.

[bbondy]

Posted a follow up for this here #20246

[srirambv]

Brave	1.35.98 Chromium: 98.0.4758.72 (Official Build) (64-bit)
Revision	d0fe1ec4df090cd3eb02b591228505e12ea476e9-refs/branch-heads/4758@{#935}
OS	☑️ Linux	☑️ Windows 11 Version Dev (Build 22523.1000)	☑️ macOS Version 12.0.1 (Build 21C52)
Verified steps from issue description Verified able to view addresses, transactions, and contracts Verified able to open block explorer links both from Wallet page as well as panel Verified able to open block explorer links for custom networks, such as Polygon
	20192-Linux.mp4	20192-Windows.mov	20192-macOS.mov