[otap-df-otap] Restructure Syslog CEF Receiver config to scope settings per protocol

[utpilla]

Change Summary

Restructures the Syslog CEF Receiver configuration so that endpoint and TLS settings are scoped under their respective protocol, rather than being flat top-level fields. This aligns with the OTLP Receiver's config pattern and makes the config model extensible for future protocol additions (e.g., Unix Domain Sockets).

Motivation

The previous config had listening_addr, protocol, and tls as sibling fields:

• TLS is inherently TCP-specific, but nothing in the config structure enforced that
• A flat listening_addr (SocketAddr) wouldn't generalize to protocols which might be added in future such as Unix Domain Sockets that use a path instead
• Adding new protocols would require awkward conditional validation

Config change

Before

Config
├── listening_addr: SocketAddr
├── protocol: "tcp" | "udp"
└── tls: Option<TlsServerConfig>     ← TCP-only, but not enforced by structure

After

Config
└── protocol: Protocol                ← tagged enum, exactly one variant
    ├── tcp: TcpConfig
    │   ├── listening_addr: SocketAddr
    │   └── tls: Option<TlsServerConfig>
    └── udp: UdpConfig
        └── listening_addr: SocketAddr

YAML example

Before

config:
  listening_addr: "0.0.0.0:5140"
  protocol: udp

After

config:
  protocol:
    udp:
      listening_addr: "0.0.0.0:5140"

Key design decisions

• Tagged enum instead of struct with optional fields: Unlike the OTLP Receiver (which supports running gRPC + HTTP simultaneously), the Syslog CEF Receiver supports exactly one protocol per instance. A serde externally-tagged enum enforces this at deserialization time with no runtime validation needed.
• protocol (singular): Reflects the one-protocol-per-instance constraint.
• TLS scoped to TcpConfig: UDP's deny_unknown_fields rejects any tls field, enforcing the constraint structurally.

How are these changes tested?

Added unit tests

Are there any user-facing changes?

Yes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.97%. Comparing base (74b09ca) to head (2c8aaa7).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2064      +/-   ##
==========================================
+ Coverage   86.96%   86.97%   +0.01%     
==========================================
  Files         536      536              
  Lines      172511   172637     +126     
==========================================
+ Hits       150020   150152     +132     
+ Misses      21957    21951       -6     
  Partials      534      534              

Components	Coverage Δ
otap-dataflow	89.12% <100.00%> (+0.01%)	⬆️
query_abstraction	80.61% <ø> (ø)
query_engine	90.31% <ø> (ø)
syslog_cef_receivers	∅ <ø> (∅)
otel-arrow-go	53.50% <ø> (ø)
quiver	91.73% <ø> (ø)

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[drewrelmas]

Approved - please take a quick look at the failing Clippy notice.

[cijothomas]

Nice!

[lquerel]

LGTM