Add okhttp dep to r5 and indicate which deps need to be included

[tadgh]

• Currently building javadocs in R5 is failing, causing a failed deploy to sonatype:

[ERROR] /__w/1/s/hapi-fhir-structures-r5/target/distro-javadoc-sources/org.hl7.fhir.r5-5.6.27-sources/org/hl7/fhir/r5/utils/client/network/FhirLoggingInterceptor.java:4: error: package okio does not exist
[ERROR] import okio.Buffer;
[ERROR]            ^

• Failure can be replicated by cleaning your ~/.m2 and running mvn clean install -P DIST in the hapi-fhir-structures-r5 directory
• This MR makes the R5 pom.xml match the structure of R4 and DSTU3, which import the lib as an addditional dep during javadoc generation.

[codecov]

Codecov Report

Merging #3332 (48f47cc) into master (31bb1d9) will increase coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #3332   +/-   ##
=========================================
  Coverage     82.80%   82.81%           
- Complexity    20460    20463    +3     
=========================================
  Files          1367     1367           
  Lines         73521    73521           
  Branches      11053    11053           
=========================================
+ Hits          60882    60886    +4     
+ Misses         8390     8388    -2     
+ Partials       4249     4247    -2     

Impacted Files	Coverage Δ
.../uhn/fhir/jpa/term/TermDeferredStorageSvcImpl.java	89.20% <0.00%> (-0.94%)	⬇️
...or/TransactionConcurrencySemaphoreInterceptor.java	85.29% <0.00%> (+1.47%)	⬆️
...r/UserRequestRetryVersionConflictsInterceptor.java	87.50% <0.00%> (+4.16%)	⬆️
...scription/match/deliver/email/EmailSenderImpl.java	100.00% <0.00%> (+25.00%)	⬆️

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