Upgrade proxy agents

[OscarBarrett]

Resolves #1155
Related to #1169

Summary of changes

• Upgraded http-proxy-agent to ^5.0.0
• Upgraded https-proxy-agent to ^5.0.1
• Removed replaced types: @types/http-proxy-agent (types are now self contained)
• Removed replaced type declaration for https-proxy-agent
• Removed use of new for agent initialization
• Work around private property access in tests by using string index notation for agent['proxy']

[OscarBarrett]

FYI: My reason for PRing this is separate from the linked issue, but is due to incompatibility with node 16 on a custom Bitbucket Pipelines Runner.

The current version errors with:

class HttpProxyAgent extends agent_base_1.Agent {
                                          ^
TypeError: Class extends value undefined is not a constructor or null
    at Object.<anonymous> (/opt/atlassian/pipelines/agent/build/node_modules/http-proxy-agent/dist/agent.js:31:43)
    at Module._compile (node:internal/modules/cjs/loader:1126:14)
    ...

[orta]

👍🏻 this looks good to me, thanks!