Add a switch for IPFS companion extension

[yrliou]

Test plan

See brave/brave-core#2106

Description

Add a pref to control IPFS companion extension installation, which will be off by default.

[rebron]

cc: @Brave-Matt We're going to need an article to explain this switch and IPFS in general.

[bsclifton]

Updated top post to link to test plan 😄
cc: @yrliou

[btlechowski]

Verification passed on

Brave	0.63.45 Chromium: 74.0.3729.75 (Official Build) beta (64-bit)
Revision	fdb7915642fef8cf997beac2554709d148e3c187-refs/branch-heads/3729@{#754}
OS	Windows 10 OS Build 17134.523

Used test plan from brave/brave-core#2106
verified that user cannot install ipfs extension from Google webstore when ipfs is already installed through the toggle

Logged #4211 and #4212
Encountered while testing #3950

Verification passed on

Brave	0.64.68 Chromium: 74.0.3729.108 (Official Build) beta (64-bit)
Revision	daaff52abef89988bf2a26091062160b1482b108-refs/branch-heads/3729@{#901}
OS	Linux

• Verified the test plan from Ipfs companion switch brave-core#2106 - Test plan needs to edited IPFS companion extension is not removed from brave://components #4211 (comment)
• Verified that IPFS isn't component extension and won't be displayed under brave://components anymore IPFS companion extension is not removed from brave://components #4211 (comment)
• Verified that when IPFS is enabled and installed, It will be displayed and enabled under brave://extensions
• Verified that when IPFS is disabled in brave://settings, It will be disabled under brave://extensions

Verified passed with

Brave	0.64.72 Chromium: 74.0.3729.131 (Official Build) beta(64-bit)
Revision	518a41c1fa7ce1c8bb5e22346e82e42b4d76a96f-refs/branch-heads/3729@{#954}
OS	Mac OS X

• Verified the test plan from Ipfs companion switch brave-core#2106 - Test plan needs to edited IPFS companion extension is not removed from brave://components #4211 (comment)
• Verified that IPFS isn't component extension and won't be displayed under brave://components anymore IPFS companion extension is not removed from brave://components #4211 (comment)
• Verified that when IPFS is enabled and installed, It will be displayed and enabled under brave://extensions
• Verified that when IPFS is disabled in brave://settings, It will be disabled under brave://extensions