Add API to select which types of ACME challenges will be used

[natemcmaster]

The ACME protocol supports multiple methods for proving you own a DNS name called "challenge types". If you wish to manually select which challenge types are used, set the "AllowedChallengeTypes" method. The default value is "Any", which means this library will exhaust all supported challenge types before giving up.

Current supported values:

• Http01 - The HTTP-01 challenge, which uses a well-known URL on the server and a HTTP request/response.
• TlsAlpn01 - The TLS-ALPN-01 challenge, which uses an auto-generated, ephemeral certificate in the TLS handshake.
• Any - (default) - use HTTP-01 and/or TLS-ALPN-01

Tip: if you wish to set multiple method types and are use the "appsettings.json" approach, provide a comma-seperated list.

{
    "LettuceEncrypt": {
        "AllowedChallengeTypes": "Http01, TlsAlpn01"
    }
}

[codecov[bot]]

Codecov Report

Merging #197 (5e7574f) into main (ae682e9) will decrease coverage by 0.24%. The diff coverage is 21.42%.

@@            Coverage Diff             @@
##             main     #197      +/-   ##
==========================================
- Coverage   47.16%   46.92%   -0.25%     
==========================================
  Files          41       41              
  Lines        1060     1072      +12     
==========================================
+ Hits          500      503       +3     
- Misses        560      569       +9     

Impacted Files	Coverage Δ
.../LettuceEncrypt/Internal/AcmeCertificateFactory.cs	0.00% <0.00%> (ø)
...ttuceEncrypt/Internal/TlsAlpnChallengeResponder.cs	21.73% <66.66%> (+3.55%)	:arrow_up:
src/LettuceEncrypt/LettuceEncryptOptions.cs	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update ae682e9...5e7574f. Read the comment docs.